PABLO DANIEL
BENÍTEZ MONGELÓS
Estratégias de apoio à manutenção de estruturas de
betão armado com risco de corrosão
Maintenance support strategies for reinforced
concrete structures under corrosion risk
Ano 2018
PABLO DANIEL
BENÍTEZ MONGELÓS
Estratégias de apoio à manutenção de estruturas de
betão armado com risco de corrosão
Tese apresentada à Universidade de Aveiro para cumprimento dos requisitos necessários à obtenção do grau de Doutor em Engenharia Civil, realizada sob a orientação científica da Doutora Maria Fernanda da Silva Rodrigues, Professora Auxiliar do Departamento de Engenheira Civil da Universidade de Aveiro, e coorientação científica do Doutor Humberto Salazar Amorim Varum, Professor Catedrático da Faculdade de Engenharia da Universidade do Porto.
Ano 2018
PABLO DANIEL
BENÍTEZ MONGELÓS
Maintenance support strategies for reinforced
concrete structures under corrosion risk
This thesis is submitted to the University of Aveiro to fulfil the necessary requirements for the degree of Doctor of Philosophy in Civil Engineering, performed under the scientific supervision of Maria Fernanda Rodrigues, Assistant Professor of the Department of Civil Engineering at the University of Aveiro, and the co-supervision of Humberto Salazar Amorim Varum, Full Professor of the Department of Civil Engineering at the University of Porto.
This research has been funded with the support of the European Commission. This publication reflects the view only of the author, and the Commission cannot be held responsible for any use, which may be made of the information contained therein - ELARCH program (Project Reference number: 552129-EM-1-2014-1-IT-ERA MUNDUS-EMA21)
o júri
presidente Doutor Aníbal Guimarães da Costa
Professor Catedrático, Universidade de Aveiro
vogais Doutor Enrico Spacone
Professor Catedrático, Università Degli Studi "Gabrielle d'Annunzio" Chieti-Pescara
Doutor Hugo Filipe Pinheiro Rodrigues Professor Adjunto, Instituto Politécnico de Leiria Doutor José Filipe Miranda Melo
Investigador, Faculdade de Engenharia da Universidade do Porto Doutor Eugénio Alexandre Miguel Rocha
Professor Auxiliar, Universidade de Aveiro
Doutora Maria Fernanda da Silva Rodrigues
the jury
chairman Doctor Aníbal Guimarães da Costa
Full Professor, University of Aveiro Doctor Enrico Spacone
Full Professor, University "Gabrielle d'Annunzio" of Chieti-Pescara Doctor Hugo Filipe Pinheiro Rodrigues
Associate Professor, Polytechnic Institute of Leiria Doctor José Filipe Miranda Melo
Researcher, Faculty of Civil Engineering of University of Porto Doctor Eugénio Alexandre Miguel Rocha
Assistant Professor, University of Aveiro Doctor Maria Fernanda da Silva Rodrigues Assistant Professor, University of Aveiro
agradecimentos /
acknowledgements I would like to express my gratitude in the first place to Professor Fernanda Rodrigues, my supervisor, for her continuous encouragement and wholehearted support that has been invaluable in the development of this thesis. Her dedication, friendship, constructive discussion, endless patience and generosity have contributed to making this research possible and help me to develop my capabilities as a researcher.
I also would like to express my appreciation to Professor Humberto Varum, my co-supervisor, for always giving me their advice and suggestions, and for providing me with materials that helped to understand better the technical concepts developed in this research.
I am especially grateful to Professor Sudip Talukdar, whose recommendations and constructive criticism have helped me to develop and better understand the modelling of carbonation in structures. His extensive knowledge and comments to my work have allowed providing an improved quality to the results obtained in this study.
I would like to thank Professor Eugenio Rocha for his availability, commitment and dedication in the elaboration of the numerical models developed in part of this work. The exchange of their knowledge and vision have helped to develop a large part of the scientific contributions of this thesis.
To Professor Sergio Gavilan, who give me the support and all the information necessary to perform the analysis of carbonation in Paraguay. His collaboration has been fundamental to be able to frame the study in the context outlined in the objectives of this thesis.
I would like to acknowledge to Professor Enrico Spacone for his help and guidance in the first years of the thesis. Also, I would like to thank him for the support provided in the conditions offered during my stay at University G. D'Annunzio.
To all the workers of the Department of Civil Engineering, for their kindness and patience from my first days at the University. Also, I express a special thanks to the colleagues of the doctorate room, for giving me their friendship and support in these years. I am grateful for making me feel at home.
Last but not least, I would like to express my gratitude to my wife Fabiana for her support, motivation, patience and love. Her understanding from the first moment has made possible the realisation of this work. I would also like to thanks to my parents, Carlos and Maura, for always encouraging me to reach my goals. Their dedication and responsibility have been a source of inspiration to complete this work. And to all the people who have indirectly collaborated with this thesis and for their support in these three years.
palavras-chave degradação do betão, alterações climáticas, carbonatação, vida útil, análise de eficiência, dados reais de carbonatação, otimização de manutenção, tomada de decisão, manutenção preventiva.
resumo As estruturas de betão armado (BA) constituem uma grande parte das estruturas
e infraestruturas construídas em todo o mundo. A sua maioria foi construída na primeira metade do século passado, logo, a sua vida útil apresenta atualmente, em muitos casos, um estado crítico, na perspetiva da sua manutenção. Um dos mecanismos de degradação mais frequentes e dispendiosos neste tipo de estruturas está associado à corrosão das armaduras. Esta tese analisa a corrosão induzida por carbonatação, considerando que é a principal causa de degradação em estruturas de betão no Paraguai.
O problema crescente advindo das alterações climáticas alertou os responsáveis da manutenção deste tipo de estruturas, desde o final do século passado. Vários estudos mostraram que podem ser afetadas pelo impacto desse fenômeno no que respeita à sua durabilidade, salientando a redução na expectativa de vida útil dessas estruturas, causada por um aumento da taxa de corrosão, associada ao aumento da temperatura e às concentrações atmosféricas de CO2. Estas conclusões levaram ao desenvolvimento desta tese, cujo objetivo principal é desenvolver uma metodologia otimizada para a formulação de estratégias de manutenção de estruturas de BA submetidas à degradação por corrosão induzida por carbonatação, considerando os efeitos das alterações climáticas.
Para o cumprimento dos objetivos desta tese, efetuou-se uma análise de modelos numéricos de carbonatação em estruturas de betão armado, para a obtenção das curvas de degradação. As curvas de degradação obtidas com o modelo matemático escolhido e modificado, mostram a profundidade de carbonação esperada no Paraguai para os próximos 50 anos, considerando diferentes cenários climáticos. Por sua vez, com esta análise, foi possível determinar os tempos de início e de propagação da corrosão previstos para as estruturas, considerando diversas configurações dos principais parâmetros que influenciam a durabilidade: a qualidade do betão e a espessura do recobrimento. Definidas as condições de degradação foram formuladas estratégias de manutenção preventiva baseadas em modelos de decisão. Estes modelos numéricos foram estabelecidos em duas etapas que compreendem o planeamento das inspeções e o planeamento das reparações. Para a primeira etapa foi proposta uma análise de eficiência, complementada com o processo de otimização dos momentos de inspeção e das técnicas de intervenção mais apropriadas. Para o planeamento da reparação, foi desenvolvido um modelo dinâmico para apoio à tomada de decisões que considera a análise de custos da estratégia de manutenção e a capacidade das técnicas de inspeção e reparação para garantir a durabilidade das estruturas, de forma eficaz, através de manutenção preventiva.
Os resultados desta tese mostram que o risco de degradação de estruturas de betão armado induzidas por carbonatação, pode aumentar no futuro devido ao efeito das alterações climáticas. Assim, para o pior cenário climático, estima-se um aumento médio de 25% na profundidade máxima de carbonação na segunda metade deste século em relação a um cenário de controle. Enquanto que o tempo para atingir a mesma profundidade máxima de carbonatação do cenário de controle pode ser reduzido entre 7 e 10 anos para o melhor cenário, dependendo da qualidade do betão. Além disso, o modelo de manutenção desenvolvido é facilmente aplicável e permite a formulação de estratégias de longo prazo que otimizem recursos ao menor custo, para lidar com esse mecanismo de degradação.
keywords concrete degradation, climate change, carbonation, service life, efficiency analysis, real carbonation data, maintenance optimisation, decision-making, preventive maintenance.
abstract Reinforced concrete (RC) structures comprise a large part of the structures and infrastructures around the world. Most of them have been built in the first half of the 20th-century, so their service life is currently at a critical stage from the maintenance perspective. One of the most frequent and expensive degradation mechanisms they present is associated with the reinforcement corrosion. This research is focused on the carbonation-induced corrosion considering that it is the primary degradation cause of RC structures in Paraguay.
The growing problem of climate change has caught the attention of maintenance managers of these structures since the end of the last century. Several studies shown that structures can be affected by the impact of this phenomenon considering its durability highlighting the reduction of its expected service life caused by an increase in the corrosion rate associated with the rise in temperature and the atmospheric concentrations of CO2. These conclusions led to the development of this thesis whose main objective is to develop an optimised methodology for the formulation of maintenance strategies of RC structures subjected to the carbonation-induced corrosion, considering the effects of climate change.
For the fulfilment of the objectives of this thesis, an analysis has been developed on the numerical modelling of carbonation in RC structures to obtain the degradation curves. The degradation curves obtained by the chosen and modified model show the expected carbonation depth in Paraguay for the next 50 years under the consideration of different climate scenarios. With this analysis, it was possible to determine the corrosion initiation and corrosion propagation times for the structures considering several configurations of the principal parameters influencing durability: the quality of the concrete and the cover thickness.
Once the degradation conditions were defined, preventive maintenance strategies based on decision models were formulated. These numerical models were established in two stages comprising the inspections and the repairs planning. For the inspections planning, an efficiency analysis was proposed that is complemented by the optimisation process of the inspection schedule and the most appropriate intervention techniques. For the repairs planning, a dynamic model for decision-making has been developed which considers the cost analysis of the maintenance strategy and the capabilities of the inspection and repair techniques to effectively ensure the durability of the structures through the preventive maintenance approach.
The main results of this thesis shown that the carbonation-induced corrosion risk of structures can increase in the future due to the climate change effect. Thus, for the worst climate scenario, in the second half of this century is expected an increase by 25% in the maximum carbonation depth regarding a control scenario. Meanwhile, the time to reach the same maximum carbonation depth of the control scenario can be reduced between 7 and 10 years for the best climate scenario, depending on the quality of the concrete. Furthermore, the maintenance model developed in this research is easily applicable and allows the formulation of long-term strategies that optimise resources at the lowest cost to deal with this degradation mechanism.
palabras claves degradación del concreto, cambio climatico, carbonatación, vida útil, análisis de eficiencia, datos reales de carbonatacion, optimización del mantenimiento, toma de decisiones, mantenimiento preventivo.
resumen Las estructuras de hormigón armado (H°A°) comprenden una gran parte de las
infraestructuras en todo el mundo. La mayoría de estas estructuras se han construido en la primera mitad del Siglo XX, por lo que su vida útil se encuentra actualmente en una etapa crítica desde la perspectiva del mantenimiento. Uno de los mecanismos de degradación más frecuentes y costosos en este tipo de infraestructura está asociado con la corrosión del refuerzo. Esta investigación se centra en la corrosión inducida por la carbonatación, ya que es la principal causa de degradación de las estructuras de H°A° en Paraguay.
El creciente problema del cambio climático ha llamado la atención de los administradores del mantenimiento de las estructuras desde finales del siglo pasado. Varios estudios han demostrado que las infraestructuras podrían verse afectadas por el impacto de este fenómeno desde el punto de vista de la durabilidad. Estos estudios han demostrado una reducción en la vida útil prevista de las estructuras causada por un aumento en la tasa de corrosión asociada con el aumento de temperatura y las concentraciones atmosféricas de CO2. Esta condición ha llevado a la elaboración de esta tesis cuyo principal objetivo es desarrollar una metodología optimizada para la formulación de estrategias correspondientes al mantenimiento de estructuras H°A° sometidas a la corrosión inducida por la carbonatación, considerando los efectos del cambio climático. Para el cumplimiento de los objetivos de esta tesis, se ha desarrollado un análisis sobre la modelación numérica de la carbonatación en el hormigón para obtener las curvas de degradación en estas estructuras. Estas curvas muestran la profundidad de carbonatación esperada en Paraguay durante los próximos 50 años bajo la consideración de diferentes escenarios climáticos. A su vez, con este análisis se ha podido determinar el tiempo de inicio y propagación de la corrosión previstos para las estructuras considerando varias configuraciones de los parámetros principales para la durabilidad: la calidad del hormigón y el espesor del recubrimiento.
Una vez que se han definido las condiciones de degradación, se han formulado estrategias de mantenimiento preventivo basadas en modelos de decisión. Estos modelos numéricos se han establecido en dos etapas que comprenden la planificación de inspecciones y la planificación de reparaciones. Para la planificación de las inspecciones, se ha propuesto un análisis de eficiencia que se complementa con el proceso de optimización de los tiempos de inspección y las técnicas de intervención más adecuadas. Para la planificación de reparaciones, se ha desarrollado un modelo dinámico para la toma de decisiones que considera el análisis de costos de la estrategia de mantenimiento y las capacidades de las técnicas de inspección y reparación para garantizar la durabilidad de las infraestructuras de manera efectiva a través del enfoque de mantenimiento preventivo.
Los resultados de esta tesis han demostrado que el riesgo de corrosión inducida por la carbonatación de las estructuras podría aumentar en el futuro debido al efecto del cambio climático. Así, para el peor escenario climático, podría esperarse un aumento medio de 25% de la profundidad máxima de carbonatación en la segunda mitad de este siglo respecto a un escenario de control. Mientras tanto, el tiempo para alcanzar la misma profundidad máxima de carbonatación del escenario de control podría reducirse entre 7 y 10 años para el mejor escenario climático, dependiendo de la calidad del hormigón. Además, el modelo de mantenimiento desarrollado en esta investigación es fácilmente aplicable y permite la formulación de estrategias a largo plazo que optimizan los recursos al menor costo para hacer frente a este mecanismo de degradación.
I
CONTENTS
CONTENTS ... I LIST OF FIGURES ... V LIST OF TABLES ... VII SYMBOLOGY ... IX ACRONYMS ... XV
1 INTRODUCTION ... 3
1.1 Context of the Research ... 3
1.1.1 The Climate Change ... 4
1.1.2 Degradation in Concrete Structures ... 5
1.1.3 Modelling of Optimal Maintenance ... 6
1.2 Significance and Methodology of the Research ... 8
1.3 Research Objectives ... 10
1.4 Outline of the Research ... 12
1.5 Publications associated with this Thesis ... 13
2 CLIMATE CHANGE ... 17
2.1 Introduction ... 17
2.2 Scientific basis of Climate Change ... 18
2.3 Climate Change Effects ... 22
2.4 Trends of Climate Change ... 24
2.5 Climate Change in the context of Paraguay ... 26
2.6 Effects in the Infrastructures ... 28
2.7 Summary ... 30
3 DEGRADATION OF REINFORCED CONCRETE ... 35
3.1 Introduction ... 35
3.2 Durability of Concrete Structures: Influential factors ... 36
3.3 Degradation Mechanisms ... 39
3.3.1 Sulphates Attack ... 41
3.3.2 Abrasion and Crystallisation ... 41
3.3.3 Temperature variations - Freeze/Thaw Cycle ... 42
3.3.4 Alkali-Silica Reaction ... 42
Contents
II
3.4 The Corrosion in Concrete Structures ... 43
3.4.1 Carbonation ... 45
3.4.2 Chloride penetration ... 48
3.5 Parameters that influence Carbonation-induced Corrosion ... 49
3.5.1 Water/Cement Ratio ... 50
3.5.2 Cover Thickness ... 50
3.5.3 Type of Binder ... 51
3.5.4 Concrete execution ... 51
3.5.5 Environmental factor ... 52
3.6 Inspection Techniques and Maintenance ... 53
3.6.1 Inspection Techniques ... 54
3.6.2 Maintenance Techniques ... 58
3.7 Carbonation in Latin America ... 60
3.7.1 Carbonation in Structures of Paraguay ... 63
3.8 Summary ... 67
4 MODELLING OF CARBONATION-INDUCED CORROSION ... 73
4.1 Introduction ... 73
4.2 Modelling the Carbonation-induced Corrosion ... 77
4.3 Carbonation model adopted ... 83
4.3.1 Corrosion Initiation Stage... 84
4.3.2 Corrosion Propagation Stage ... 88
4.4 Degradation curves for Structures in Paraguay ... 92
4.4.1 Parameters considered for the simulation ... 92
4.4.2 Results of the numerical simulation ... 94
4.4.3 Correlation Analysis of the results ... 98
4.5 Summary ... 101
5 MAINTENANCE STRATEGIES FOR RC STRUCTURES ... 107
5.1 Introduction ... 107
5.2 Optimal Inspection Planning ... 109
5.2.1 Efficiency Analysis ... 110
5.2.2 Mathematical Formulation for the Inspection Planning ... 112
5.2.3 Application of the Methodology ... 122
5.2.4 Discussion of results ... 126
5.3 Dynamic Decision-Making Model ... 129
III
5.3.2 Dynamic Maintenance Model for RC Structures ... 133
5.3.3 Results of the Methodology ... 141
5.3.4 Analysis of study ... 145
5.4 Summary ... 148
6 CONCLUSIONS... 153
6.1 Final remarks ... 153
6.2 Main Conclusions ... 153
6.3 Suggestions for further Research ... 156
V
LIST OF FIGURES
Figure 1.1 Representation of the basic elements of a maintenance model ... 7 Figure 1.2 Worldwide Cost of Corrosion. ... 9 Figure 1.3 Formulation scheme of the support decision model for the maintenance of structures. 10 Figure 1.4 Flowchart with activities to develop research. ... 11 Figure 2.1 Global atmospheric concentration of CO2 over the last decades ... 19
Figure 2.2 Global energy flow in the Earth by the period 2000-2004 ... 19 Figure 2.3 Trends of the global surface warming increase in the coming years. ... 24 Figure 2.4 Expected CO2 concentration for the RCPs scenarios. ... 25
Figure 2.5 Variation of temperature and RH in Asunción ... 26 Figure 2.6 Expected temperature increment in Paraguay for the scenarios RCP 4.5 and RCP 8.5. 27 Figure 3.1 Performance of RC structures over its service life... 38 Figure 3.2 Degradation function for a constructive element of a building concerning its expected life-cycle. ... 40 Figure 3.3 The corrosion process in RC structures. ... 44 Figure 3.4 Pourbaix diagram for corrosion. ... 45 Figure 3.5 Degradation zones of a carbonated concrete structure ... 46 Figure 3.6 Corrosion sequence in RC structures. ... 46 Figure 3.7 Carbonation depths concerning relative humidity ... 53 Figure 3.8 Some structures of the case study ... 64 Figure 3.9 Normal distribution of test results. ... 64 Figure 3.10 Evaluation of Carbonation depth through phenolphthalein tests. ... 65 Figure 3.11 Measurement of cover thickness in the structures. ... 65 Figure 3.12 Scheme established for corrosion risk analysis. ... 66 Figure 3.13 Degradation conditions of structures based on inspections results. ... 67 Figure 4.1 Relationship among the models for the prediction of service life. ... 73 Figure 4.2 Stages and sub-stages in the service life of corroded concrete structures. ... 75 Figure 4.3 Flowchart of the carbonation model applied in this research. ... 83 Figure 4.4 Flowchart for the concentrations determination of CO2 and Ca(OH)2 as a function of
time ... 84 Figure 4.5 Progress of corrosion-induced damage in concrete. ... 89 Figure 4.6 Modelling of propagation stage of corrosion. ... 89
List of Figures
VI
Figure 4.7 Köppen-Geiger Climate classification map of Paraguay. ... 93 Figure 4.8 Expected carbonation depths for RC structures in Asunción until 2077 ... 95 Figure 4.9 Projections trends for CO2 emissions and temperature ... 96
Figure 4.10 Correlation between real carbonation data and carbonation curves of the model. ... 99 Figure 5.1 Proposed framework for the formulation of maintenance strategies. ... 108 Figure 5.2 Production Frontiers and Technical Efficiency ... 110 Figure 5.3 Implementation of MEA model for efficiency analysis ... 112 Figure 5.4 Flowchart for the inspection planning formulation. ... 113 Figure 5.5 Event tree for the probability of early detection of damage. ... 118 Figure 5.6 Probability Density Function for (a) 𝑇 and (b) 𝑇 . ... 123 Figure 5.7 Cumulative Density Function for the Detectability regarding the damage degree ... 124 Figure 5.8 Cumulative Density Function for the Detectability over the service life time. ... 125 Figure 5.9 Boundary points of damage degree for each inspection techniques. ... 125 Figure 5.10 General process of a MCDM model. ... 130 Figure 5.11 Hierarchical structure for decision-making. ... 132 Figure 5.12 Scheme formulated for repair methods. ... 135 Figure 5.13 Capabilities of the inspection and repairs over the service life. ... 143 Figure 5.14 Global priorities of maintenance alternatives over time. ... 144
VII
LIST OF TABLES
Table 2.1 Historical progress of climatic scenarios ... 21 Table 2.2 Types of representative concentration pathways. ... 21 Table 2.3 Effects of climate change on infrastructures. ... 29 Table 3.1 Interpretation of resistivity test results. ... 56 Table 3.2 Interpretation of resistivity test results. ... 57 Table 3.3 Summary of results obtained from the intervention of concrete buildings in Asunción . 65 Table 4.1 Classification of corrosion propagation models. ... 76 Table 4.2 Progression of modelling of carbonation-induced corrosion. ... 77 Table 4.3 Values of the variables applied in the carbonation model ... 91 Table 4.4 Expected Service life for RC structures in Asunción. ... 97 Table 5.1 Parameters for the generation of 𝑇 and 𝑇 . ... 122 Table 5.2 Parameters estimated for the inspection techniques. ... 124 Table 5.3 Outcomes of the Efficiency Analysis for the Optimal Inspection Planning. ... 126 Table 5.4 Best inspection sequences for each variable considered. ... 128 Table 5.5 Optimal inspection planning for 𝑉 = 0.015 𝑐𝑚/𝑦𝑒𝑎𝑟. ... 129 Table 5.6 Fundamental scale for verbal judgements. ... 132 Table 5.7 Values for random index. ... 133 Table 5.8 Stochastic indexes for each criterion. ... 140 Table 5.9 Comparison matrix of alternatives. ... 140 Table 5.10 Example for the final pairwise comparison matrix between criteria and alternatives. 141 Table 5.11 Random variables considered for the study. ... 142 Table 5.12 Repairs method considered for the analysis. ... 142 Table 5.13 Pairwise comparison matrix of criteria 𝐶𝑅 = 0.074. ... 143 Table 5.14 Best alternatives of intervention for the maintenance planning of structures ... 144
IX
SYMBOLOGY
A Vector for the set of alternatives in the decision problem 𝐴 Empirical parameter for effective diffusion of CO2
𝑎 Individual weights of each alternatives in the comparison matrix b Cover thickness of concrete
c Cement content
Ca(OH)2 Calcium Hydroxide
CaCO3 Calcium carbonate
CI Consistency Index
CO2 Carbon Dioxide
CR Consistency Ratio
C-S-H Calcium-Silicate-Hydrate
𝐶 Consistency factor for the proposed comparison matrix 𝐶 Initial construction cost
𝐶𝑁𝑃𝑉𝛾 Present cost for applying the repair method in the future 𝐶𝑁𝑃𝑉𝜃 Present cost for applying the inspection technique in the future
𝐶𝑂 ( ) Aqueous carbon dioxide solution
𝐶𝑂 ( ) Atmospheric concentration of CO2
𝐶𝑎(𝑂𝐻) ( ) Aqueous concentration of calcium hydroxide
𝐶 Air content coefficient
𝐶 Carbonation depth
𝐶 Environmental coefficient
𝐶𝑖𝑛𝑠𝑝𝜃
Unit cost of the inspection technique 𝜃
𝐶𝑟𝑒𝑝𝛾 Unit cost of the repair method 𝛾
𝐶 Cost of the inspection sequence 𝜌 𝐷 Effective diffusion of CO2
𝐷 Diffusion coefficient of reference
D(T) Diffusion of CO2 as a function of temperature
D Global effective diffusion of CO2
Symbology
X
𝑑 Initial diameter of the reinforcement 𝑑(𝑡) Diameter of the reinforcement over time 𝐸 Modulus of elasticity of concrete at 28 days 𝐸 Effective modulus of elasticity of concrete 𝑒𝑓𝑓𝑅 Efficiency of the repair method
𝑓 Characteristic compressive strength of concrete 𝑓 Yield stress of concrete
F Faraday’s constant
𝐺(𝑋) Generic limit state function
𝑔(𝜂(𝑡)) Limit state function for damage degree
H Henry constant
Href Reference Henry’s constant
𝐼(𝑡) Corrosion current density over time
𝐼 ; 𝐼 Stochastic indexes for the decision-making analysis I Set of inputs for the efficiency analysis
Io Current flow density at the reference temperature
J Set of outputs for the efficiency analysis 𝐾 Solubility product of calcium hydroxide
𝐾 Coefficient of degradation improvement given by the repair method 𝑘 Reaction rate constant between CO2 and Ca(OH)2
L Longitudinal dimension of the concrete specimen 𝐿𝑜𝑔𝑁 Log-Normal distribution
m Humidity constant
M Molar mass of the steel rebar
𝑀𝐸𝐴(𝜌) Multidirectional efficiency analysis score for an inspection sequence 𝜌
𝑁 Number of different inspection techniques necessary for the inspection sequence 𝑂 Optimisation problem for the best inspection sequence
𝑝 ; 𝑝 Polynomials for the detectability expression
𝒫 Proposed expression for the detectability of the inspection technique 𝑃 Probability of damage detection before failure
𝑃 (𝜂(𝑡)) Probability of Doing Repair
𝑃 Probability of damage detection of the inspection technique 𝜃
𝑃𝐷 Maximum probability of damage detection before failure attained by the inspection sequence 𝜌
XI 𝑃 Probability of improve the damage condition in the structure
𝑃𝑓 Probability of failure
𝑃 (𝜌̅); 𝑃 (𝜌̅) Linear programming optimisation problems for the efficiency analysis 𝑃 (𝛼, 𝛽, 𝜌̅) Global solution for the efficiency analysis
Q Diffusion activation energy 𝑅 Coefficient of determination R1 Gas constant for effective diffusion
R2 Gas constant for Henry’s Law
RG Design resistance for limit state function
RH Relative Humidity
RI Random Index for consistency r Annual discount rate of money 𝑆 Total inspection sequence possible
𝑆 Factor of reciprocity between the elements of the comparison matrix SG Design load for limit state function
SSTO Total sum of square
𝑆𝐹𝐴(𝜌) Stochastic frontier analysis score for an inspection sequence 𝜌 𝑆𝑆𝑅 Regression sum of squares
𝑇 Initial temperature as reference of corrosion propagation period 𝑇 Service life time
𝑇𝑓 Failure time
𝑇 Time of corrosion initiation 𝑇 Temperature of reference
𝑇 Set of the inspection sequence time T(t) Temperature for a given time T Temperature of interest
t Time
𝑡(
𝐶𝑑)
Time for attaining the 𝐶𝑡 Optimal inspection time U Reaction activation energy
𝑉 Corrosion rate
𝑉 Pore volume in the cement paste
Symbology
XII
𝑊 Time frame between the first and the last inspection time
w Water content
w/c Water/Cement ratio
𝑥(𝜌) Vector of all the inputs for the efficiency analysis 𝑦(𝜌) Vector of all the outputs for the efficiency analysis
𝑌 Minimum time admissible between consecutive inspections 𝑦 Mean value of the sample in the correlation analysis 𝑦 Measured value from real carbonation data
𝑦 Theoretical value calculated for the correlation analysis z Valence of reaction in corrosion process
𝛼 Empirical parameter for effective diffusion of CO2
𝛼 Activation energy constant
𝛼 Individual cost of the inspection technique associated with 𝜌 𝛼 Fraction of cost associated with an inspection technique
𝛼∗(𝜌); 𝛽∗(𝜌) Optimal solutions to the optimisation problems for the efficiency analysis
𝛼 Coefficient of binding agent in Häkkinen formula 𝛼𝛾 Fraction of cost associated with a repair method 𝛽 Pre-exponential factor
𝛽 Coefficient of binding agent in Häkkinen formula
β Reliability Index
𝛿 Thickness of the porous zone of concrete 𝛾 Relation between steel mass and rust
𝛾 Repair method
𝜂 . Mean damage degree associated to the repair method 𝛾
𝜂 . Damage intensity at which the inspection technique has a 50% probability of
detection
𝜂 Critical damage degree in the structure
𝜂 Maximum threshold damage degree for the detectability of the inspection technique 𝜃
𝜂 Minimum threshold damage degree for the detectability of the inspection technique 𝜃
𝜂 Maximum boundary point for the capability of the repair method 𝛾
𝜂 Minimum damage degree for the detectability of the inspection technique associated with 𝜌
𝜂 Minimum boundary point for the capability of the repair method 𝛾 𝜂 Damage degree after the repair
XIII 𝜂 Threshold damage to perform the repair in the structure
𝜂(𝑡) Corrosion damage degree over time
Λ Efficiency measurement for a number of sequence under study 𝜆 Correction factor for the consistency ratio
𝜆 Eigenvalue of the comparison matrix
𝜆 Vector of intensity variables regarding the linear combination of 𝑥(𝜌) and y(𝜌) 𝜉𝑖𝑗 Parameter for the level of importance of each alternative regarding a criterion |𝜌| Number of inspection techniques applied in the inspection sequence 𝜌 𝜌 Particular inspection sequence
𝜌 Absolute density of cement
𝜌 Inspection technique applied at the position i in the inspection sequence 𝜌 𝜌 Density of the rust
𝜌 Density of the steel 𝜌 Absolute density of water 𝜃 Set of inspection techniques
𝜎 Standard deviation of the damage degree associated to the repair method 𝛾 𝜎 Standard deviation for the detectability of the inspection technique 𝜃 𝜎 Standard deviation of the mean value
𝜓 (𝑡) Probability density function regarding the time to failure
𝜓 (𝑡) Probability density function regarding the time to corrosion initiation 𝜚 Expression for the damage parameters of the inspection technique 𝜃
𝜇 Mean value
𝜗 Poisson’s ratio of concrete
𝜛 Coefficient of maximum repair achieved by the repair method
£ British Pound
∆ Enthalpy constant
XV
ACRONYMS
ABNT Associação Brasileira de Normas Técnicas ACI American Concrete Institute
AE Acoustic Emission
AHP Analytic Hierarchy Process ANS Automated Neural network Search AR5 Fifth Assessment Report
ASTM American Society for Testing and Materials
BEM Boundary Elements Method
BIM Building Information Modelling
BS British Standard
CAF Corporación Andina de Fomento CBH Norma Boliviana de Hormigón Armado CDF Cumulative Distribution Function CEB Comite Euro-International du Betón
CEPAL Comisión Económica para América Latina y el Caribe
CIRSOC Centro de Investigación de los Reglamentos Nacionales de Seguridad para las Obras Civiles
CNCC Comisión Nacional de Cambio Climático COIN Concrete Innovation
CORDEX Coordinated Regional Climate Downscaling Experiment CPH Comisión Permanente del Hormigón
CRS Constant Returns to Scale DEA Data Envelopment Analysis
DGEEC Dirección General de Estadísticas, Encuestas y Censos DRS Decreasing Returns to Scale
EHE Instrucción de Hormigón Estructural
EN Europäische Norm
ESRL Earth System Research Laboratory FDH Free Disposability Hull
FEM Finite Elements Method
Acronyms
XVI
FRH Free Replicability Hull
GA Genetic Algorithms
GDP Gross Domestic Product GPV Global Priority Vector
HadGEM2-ES Hadley Global Environment Model 2 – Earth System IIASA International Institute for Applied Systems Analysis IPCC International Panel on Climate Change
IRS Increasing Returns to Scale
ISO International Organization for Standardization JGCRI Joint Global Change Research Institute LPR Linear Polarisation Resistance
MADM Multi Attribute Decision-Making MCDM Multi-Criteria Decision-Making MCS Monte Carlo Simulation
MEA Multidirectional Efficiency Analysis MODM Multi Objective Decision-Making
NDM Non-Destructive Methods
NDT Non-Destructive Technique
NIES National Institute for Environmental Studies NOAA National Oceanic & Atmospheric Administration
NPV Net Present Value
NSGA Non-dominated Sorting Genetic Algorithm NSR Norma Sismo Resistente
PCFV Partnership for Clean Fuels and Vehicles
PNUD Programa de las Naciones Unidas para el Desarrollo
PPM Parts Per Million
PZT Piezoelectric lead Zirconate Titanate RAC Recycled Aggregate Concrete
RC Reinforced Concrete
RCD Regional Climate Downscaling RCP Representative Concentration Pathway
RILEM International Union of Laboratories and Experts in Construction Materials, Systems and Structures
RTS Returns To Scale
SCM Supplementary Cementitious Materials SEAM Secretaría del Ambiente
XVII SFA Stochastic Frontier Analysis
SRES Special Report on Emissions Scenarios TGA Thermogravimetric Analysis
TRRL Transport and Road Research Laboratory UCD Ultimate Carbonation Depth
UNFCCC United Nations Framework Convention on Climate Change USA United States of America
USD United States Dollard VRS Variable Returns to Scale
CHAPTER 1
3
1 INTRODUCTION
1.1 Context of the Research
Reinforced concrete (RC) structures constitute a large part of all existing infrastructures throughout the world. Its successful is given by its great geometric adaptability, versatility and durability of this type of construction material. Nonetheless, the concept of concrete durability is linked to a certain period often associated with a poor design, defective construction, inadequate materials selection or a more severe environment than expected. Then, the service life is based on the ability of a structure to overcome degradation, which means that there is considerable interest from the infrastructure managers in maintaining and extending that period (Broomfield, 2007).
According to the ISO 15686-1, the service life of structures may be defined as the period of time after installation during which a building or its parts meets or exceeds the performance requirements. In these terms, the service life planning must comprise the preparation of the brief and design for the building and its parts to achieve the desired design life to reduce the costs of building ownership and facilitate maintenance and refurbishment. Therefore, when the service life of the building and its parts are estimated, maintenance planning and value engineering techniques can be applied. Likewise, the skill and expertise of the technician or organization undertaking the service life planning will be crucial to the reliability of the planning (ISO 15686-1, 2000). It should be mentioned that in this research, the term lifespan will be used to define the "age" of the structure (i.e., the time from which the structure was built) to differentiate it to the definition of service life.
In this way, the maintenance of structures has captured particular interest on the part of structural engineers from the end of the last century. Concepts and theories such as the reliability of a structure, the life cycle analysis and the adaptation of new materials to the durability requirements have been the primary approach of the maintenance studies performed so far. Therefore, to control the initial stages of degradation and prevent the failure of the structural elements, it is paramount to perform maintenance strategies. The formulation of a most cost-effective and suitable maintenance strategy can enable better budget allocation and can also minimise the decline in the performance of infrastructures during their entire life cycle (Flores-Colen and de Brito, 2010).
Currently, the challenge for engineers lies in need to ensure that structures can withstand the environmental factors that determine their degradation, which must often be considered under the constraint of a limited budget. The study of the degradation of structures caused by environmental phenomena did not reach a significant interest until a few decades ago when climate change became tangible as a critical problem that affects the daily integrity of human beings. Furthermore, such as the buildings built in earlier times (e.g., bridges, cathedrals, castles, and so on), it is now possible to classify several of the 20th-century concrete structures as modern heritage, whose maintenance has
cultural and historical value for each country.
Climate change is a highly controversial issue from the political point of view. However, regarding scientific evidence, it is a reality that cannot be avoided for a long time. This problem encourages the need to develop innovative strategies to adapt the structures to the degradation risks and guarantee their durability for as long as possible. Therefore, several studies are found in the literature regarding the optimisation of inspection and maintenance planning for infrastructures. This optimisation should seek to cover the most important aspects of a life cycle analysis such as the cost of the project, the durability, and the safety of the construction system.
Chapter 1: Introduction
4
This research thesis has as primary objective the development of an optimised maintenance methodology for concrete structures with corrosion risks by carbonation. Furthermore, this degradation risk will be analysed considering the potential effect of global climate change on the accentuation of the degradation process. The scientific methodology applied in this research is associated with the hypothetic-deductive method corresponding to the logical-theoretical approach. This method of investigation consists on the repeated observation of facts and comparable phenomena to extract the hypotheses. Then, these hypotheses are used through the inductive process of interpolation to make predictions of individual phenomena.
This thesis includes a section that addresses the problem of carbonation in concrete structures in Paraguay. This problem led to the elaboration of maintenance strategies based on decision models. These models can be used as an analysis tool for engineers working in the field of building maintenance in the country, and other countries with similar degradation problems. Therefore, the purpose of this study is to provide a numerical model to optimally formulate maintenance strategies in concrete structures under the corrosion risk of reinforcement by carbonation phenomenon. Carbonation is a chemical phenomenon that affects most of the concrete structures in the world. In the case of Paraguay, and as will be seen later, it is one of the main causes in the premature degradation of infrastructure. On the other hand, carbonation is a natural process that is directly related to environmental parameters such as the concentration of carbon dioxide (CO2), temperature
and relative humidity. Considering this approach, climate change plays a meaningful role in the future degradation of concrete structures caused by this phenomenon.
Several countries with rapidly developing infrastructure, economies or poor supervision and quality control procedures in construction have led to poor quality concrete and low concrete cover to the steel leading to carbonation problems (Broomfield, 2007). This is the case of Paraguay regarding the degradation of its infrastructures. The result of this thesis involves a first step for the development of optimised planning of the integral maintenance of buildings, which is highly necessary for structures of Paraguay. In turn, although this study focuses mainly on existing infrastructures, the results obtained concerning the advancement of carbonation can serve as a reference for the future standardisation of the cover thickness values for the concrete structures in the country.
1.1.1 The Climate Change
The Intergovernmental Panel on Climate Change (IPCC) has conducted numerous investigations through which they have concluded that much of the current warming of the earth's surface during the last 50 years is due to human activities, commonly referred to as anthropogenic activities. The climate change caused by these anthropogenic activities will probably persist for several centuries if the effects that this phenomenon produces in all aspects of daily life are not consciously taken into account (IPCC, 2007).
The climatic impact and its effects such as the increase in the intensity of rainfall, winds, temperatures and the generation of greenhouse gases cause degradation and considerable damage to the existing infrastructure every year. A study carried out in the United Kingdom indicates that the increase in wind speed by 6% is likely to cause damage to one million buildings at an approximate cost of between 1 and 2 billion pounds (£). Likewise, droughts in the summer could worsen causing an increase between 50% and 100% in the number of claims for subsidies to the state by the inhabitants in more vulnerable areas (Graves and Phillipson, 2000).
Each year, the lives of millions of people are affected when they are displaced from their residences due to climate impacts and risks. Between 2008 and 2014, an annual average of at least 22.5 million people was displaced by the direct threat or impact of floods, landslides, storms, forest fires and extreme temperatures (Yonetani, 2016). Several evaluations made by scientists have determined that many of the extreme weather events in 2011-2015, especially those related to temperature and drought, have had a considerable increase concerning the events expected because of anthropogenic
5 activities. This assumes that climate change trends are likely to have effects that would worsen in the coming years in the medium and long-term (WMO, 2016a).
In the Latin American context, and more specifically in the region corresponding to Paraguay, the situation is not encouraging. According to the Andean Development Corporation (CAF, for its acronym in Spanish), Paraguay is in the eighth position in the ranking of the countries with the highest vulnerability index against climate change. The country is classified with an "extreme" risk category, which corresponds to a high exposure index. For this reason, it is necessary that the country join efforts to develop strategies for adaptability against the durability of the structures in the face of climate change and its effects (CAF, 2014).
The climatic impact to which a building may be affected, which is exposed to a limited period of time, can be described as a stochastic process. In this way, regional climate development based on global warming can be described as a stochastic process selected from the effects generated by climate or by climate change scenarios (Lisö et al., 2003). Therefore, the study of the maintenance of structures under the consideration of climate change must entail a dynamic analysis. This analysis should allow the adaptability of the degradation models to the predicted climate conditions for the future.
This research includes a chapter where a literature review regarding general aspects of climate change such as the effects, trends and climate scenarios expected by scientists is developed. Climate change as a global phenomenon entails the development of a very broad topic. However, this thesis aims to cover an analysis only of those climatic parameters that directly affect the degradation of concrete structures due to carbonation corrosion, namely temperature, relative humidity and concentration of carbon dioxide. The durability of the building elements depends not only on its physical, chemical or mechanical properties but also on the conditions of maintenance and environmental exposure to which they are subject (Sarja et al., 2005). In this way, climate change and its effects play an important role regarding the environmental exposure that determines the durability of the elements of the infrastructures.
Extreme weather events have always been present. However, these extreme events have been changing their average values, affecting the vulnerability of infrastructures, forcing construction professionals to generate new proposals for adaptability. Consequently, climate change is a real problem which cannot be ignored, but rather it is necessary to deal with the uncertainty of the climate process. In this way, it is essential to develop methods that are increasingly optimised so that resources can be used more efficiently to ensure the durability of the construction system.
1.1.2 Degradation in Concrete Structures
Concrete is a material widely used in infrastructures since the beginning of the last century. These structures, when exposed to aggressive environments, can present processes of degradation that compromise their service life. These degradation mechanisms can be categorised according to physical, chemical, biological and structural processes. The physical processes comprise the degradation caused by exposure to extreme environmental changes such as fires, freeze/thaw cycle, among others. The chemical processes are caused by reactions between the composition of the material and the environment such as sulphate attack, alkali-acid reaction, among others. Lastly, the biological and structural processes include the presence of bacteria (mould) and the overload effects respectively (Aguirre and Gutiérrez, 2013).
Several studies may be found in the literature regarding the degradation of structures. These studies are generally focused on specific aspects such as the type of structures, the type of composite material in the construction elements, the specific degradation mechanism, among others. In this research, the study focuses specifically on the degradation of concrete structures caused by reinforcement corrosion. In turn, the corrosion process considered is centred on the corrosion caused by the carbonation of the concrete.
Chapter 1: Introduction
6
Regarding carbonation-induced degradation, the interpretation of reinforcement corrosion requires a quantitative understanding of the environment, physical deterioration process, transport mechanism through the concrete, cracking process and the corrosion phenomenon. So, development of service life prediction model for RC structures exposed to a carbonated environment is often a complicated process (Taffese and Sistonen, 2013).
The largest infrastructure problem in industrialised countries may be associated with the economic loss and damage caused by the corrosion of steel in reinforced concrete structures (Broomfield, 2007). Among the main factors that cause the corrosion of reinforcement in concrete, it is possible to identify two causes commonly presented. These are carbonation and the presence of chloride ions. Corrosion in the reinforcement causes cracking of the surface of the structure and subsequent the spalling of the cover due to the expansion of the corroded rebar. Then, the corrosion rate directly affects the extension of the service life of the RC structures (Ahmad, 2003).
One of the principal factors in the evolution of construction technologies has been motivated by the need to use materials that are capable of lasting for an extended period of time. In these terms, the end of the service life of a structure is reached when it no longer satisfies the functions for which it was designed, i.e., the loss of serviceability. Eventually, this condition can be improved through repair and maintenance activities. However, when these activities become unprofitable or complex to practice, the prolongation of service life becomes complicated. Therefore, the service life of a structure is quite dependent on the nature of the degradation mechanism to which the structure is subject (Dyer, 2014).
According to the Comite Euro-International du Betón (CEB), a truly enhanced performance cannot be achieved by improving the materials characteristics alone. Considering the complex nature of environmental effects on structures and the corresponding response, it is necessary to involve in this improvement the elements of architectural and structural design, processes of execution, and inspection and maintenance procedures, including preventive maintenance (CEB, 1989).
1.1.3 Modelling of Optimal Maintenance
Buildings and infrastructure, in general, are structures that suffer the degradation of their initial capabilities and capabilities throughout their service life. In this way, it can be said that a construction system begins to degrade immediately after its placing in service because of two independent variables: the system and the surrounding environment. However, this inevitable degradation process can be managed and controlled through periodic maintenance activities that prolong its service life (Harris, 2001; Chew et al., 2004; Rikey and Cotgrave, 2005).
The British Standard Institution defines maintenance as the combination of all technical and administrative actions, including their control, necessary for the correct operation of a certain element. These maintenance actions include operations such as cleaning, inspections, repair and replacement of building elements (BS:3811, 1984). In this context, maintenance can be established as preventive, corrective or predictive actions. Preventive maintenance refers to all actions performed on a specific schedule to keep an element in functional condition. Corrective maintenance is the unscheduled repair performed when deficiencies or failures have been perceived in order to return an element to a defined state condition, and predictive maintenance constitute the use of modern measurements methods to accurately assess condition state of the constructive element (Dhillon, 2002).
In maintenance management, the repair strategy is decided to rely on the available budget, which is part of the Life-Cycle Cost approach where the repair criterion must be treated as an optimisation parameter. Extensive studies have initiated in maintenance management due to concerns about the huge amount of investment required to maintain existing structures in service and the budget limitation that is often presented. These studies aim to identify the reasons for the observed low
7 performance of several concrete structures and afford tools that allow a more realistic assessment by means of inspections (Malioka, 2009).
The challenge of maintenance optimisation relies on that the state of conservation or performance of a structure cannot be estimated too accurately due to certain random variables that govern the mechanism of degradation. These random variables are given due to the uncertainty principle that governs all particles and phenomena of nature. This principle was developed in 1926, when the German scientist Werner Heisenberg formulated the principle of uncertainty which basically states that it is impossible to know, with high precision, the position and velocity of a particle simultaneously, without altering its natural conditions (Giribet, 2005). This makes it necessary always to admit a certain degree of uncertainty when predicting how a system will behave, which makes it quite complicated to predict the service life of a structure.
As Savage argues, when working with the prediction of the behaviour of a dataset it is common to fall into an error which he calls the "failure of the averages". This failure establishes that considering average values for the planning of activities will lead to a failure in the results. For this reason, the trend of the 21st-century is innovation in the analysis of data dealing with uncertainty, describing and
developing new methodologies and models that include simulations, decision trees, and other theories applicable to the real world (Savage, 2003).
Many numerical models can be found in the literature, which seeks to predict the degradation of the materials of a construction system by measuring its performance throughout its service life. There are also numerical models used for the planning of the maintenance of structures. These models, which will be developed later, can be elaborated according to deterministic or probabilistic methods. According to several studies the probabilistic methods are the one that best adapts to the stochastic characteristic of the degradation phenomenon.
This research proposes a decision-making model applied to the maintenance of concrete structures, which can be divided into two parts: the degradation model, and the model formulated for decision-making (Kallen, 2007). The degradation model includes a first analysis of the current state of conservation of the structure, while the decision-making model is used to predict the degradation applied to determine which strategy/policy of maintenance is optimal. Figure 1.1 shows a diagram for the development of a maintenance model.
Figure 1.1 Representation of the basic elements of a maintenance model (Kallen, 2007)
For the decision model of this thesis, planning will be formulated for the inspection and maintenance of concrete structures subject to degradation by carbonation-induced corrosion. This planning is carried out with the purpose of achieving the optimisation of the maintenance processes based on the costs of the activities, the quality of the inspection and the effectiveness of the maintenance tasks in search of the extension of its service life.
Minimise maintenance cost, corrosion initiation, and failure probability while serviceability and safety are ensured are key issues of an improved management strategy for the maintenance of RC structures. Maintenance strategies should be oriented to guarantee optimum levels of serviceability and safety during the service life of the structure, minimising both the operational costs and the environmental impact (Bastidas-Arteaga and Schoefs, 2012).
Chapter 1: Introduction
8
1.2 Significance and Methodology of the Research
The degradation of infrastructure has been influenced by several factors that over the years have been analysed by researchers to develop measures that mitigate the degradation mechanisms and prolong their service life. Nowadays, there is a factor that has become more relevant in the last decades due to the considerable effect that it causes, not only to the integrity of the buildings but also to the health and well-being of the people. This factor corresponds to the effect of climate change and global warming that planet earth is currently experiencing.
Climate change and its effects is a problem that no scientist can ignore today. There are many organisations formed in different countries that seek to adopt palliative measures that slow down the rapid effect that this phenomenon is having. To this end, representatives of the most influential countries at a global level have proposed both short-term and long-term goals that mitigate the degradation of climatic and environmental conditions. Thus, large economic investments have been developed in projects to reduce greenhouse gas emissions and implementation of energy efficiency. However, this commitment that seeks to address climate change must be carried out conscientiously and equitably in every corner of the planet. Otherwise, all the efforts made by a few will be counteracted by the indifference of others. Currently, the policies of the autonomous states directly influence the success or failure of these projects. That is, it is not possible to completely depend on these policies for an improvement in the effect of climate change. For this reason, it will be necessary to develop measures of adaptability to the consequences that climate change could bring.
The degradation and the corresponding maintenance and repair of the construction systems comprises a problem both from the viewpoint of the safety of its users and from the economic perspective. According to Balaras et al., about half of the expenses in the construction industry in Europe is oriented towards the repair, maintenance and rehabilitation of the existing buildings. The premature degradation of concrete structures is becoming the biggest problem in many countries, especially in urban zones due to adverse environmental conditions (Balaras et al., 2005). The activities related to the rehabilitation of the building stock, as a percentage of the total works concerning the building, have been continuously growing in many countries of Central Europe in the last 20 years. The long-term changes in the demand for buildings will force professionals to move their focus from new constructions to the maintenance and rehabilitation of existing buildings (Kohler and Hassler, 2002).
Regarding specifically to the corrosion, a study has shown that the estimated annual cost of corrosion worldwide exceeds the value of 1.8 trillion US dollars (USD), which translates into a 3-4 % of the Gross Domestic Product of the industrialised countries (Schmitt et al., 2009). In another more recent study, the global scale cost of corrosion was estimated even as 2.5 USD trillion (Koch et al., 2016). However, several studies estimate that between 25% and 30% of annual corrosion costs could be preserved if optimised practices in the treatment of corrosion were employed (Schmitt et al., 2009). The cost of corrosion has a significant impact on the economic aspect of countries administration worldwide. For instance, a study about cost of corrosion by sector performed on the United States in 1998 has shown that the investment in the construction sector was 50 USD billion and, likewise, the same study performed on 2012 in India has shown an investment of 8015 USD million, of which 1543 USD million was regarding to maintenance and repair activities. Nonetheless, between 15 and 35% of these expenditures could be saved if corrosion control practices available and applied to all sectors (Koch et al., 2016). Figure 1.2 shows the cost of corrosion in the Industry sector, to which the construction sector is part.
Meanwhile, regarding the carbonation-induced corrosion of concrete structures, studies show that global climate change will affect the progression of the carbonation process in infrastructures. In other words, it is expected to observe much higher carbonation depths in the long term due to the effect of climate change (Wang et al., 2010; Stewart et al., 2011). Although climate change could
9 have a minor effect in the very near future on the durability of structures, the real effects of climate change will become evident in approximately 30 years (Talukdar, 2013).
Figure 1.2 Worldwide Cost of Corrosion (Koch et al., 2016).
Paraguay being a developing country socio-economically speaking, the construction industry is experiencing significant growth since the end of the last century. However, the lack of control that occurs many times during the construction stage of the projects puts into question the quality of the constituent material of the structures. Considering its geographical location, Paraguay is a country that does not have maritime coast. This is one of the reasons why the predominant corrosion mechanism of the structures is carbonation, above the chloride attack. Nonetheless, it is not prudent to state that corrosion does not occur in the country because of the chlorides because previously it was common to use additives in the concrete mixture that possessed this chemical compound. Degradation is a term commonly related to the failure of a system, being the degradation measurements an indirect way of determining the system's failure characteristics. In other words, degradation is nothing more than the measurement of performance, quality, the capacity of the structure or damage that may occur during the lifetime. For this reason, when proposing a numerical model for the maintenance of concrete structures submitted under any failure mechanism, it is important to consider a study on its degradation firstly.
For the study of the degradation of a structure, in general, numerical models are used that make possible the elaboration of what is known as the degradation curve. This curve represents the decrease in the performance of a system throughout its service life. As such, there are innumerable numerical models of degradation in the literature based on different analytical methods. However, not all of these methods have been successfully validated or have managed to represent the degradation process reliably.
It is almost impossible, considering the uncertainty, to predict with perfect accuracy the process of degradation of a structure. However, considering the highest number of variables and possible parameters that govern a particular degradation mechanism makes a model more or less reliable. For this point, after an exhaustive bibliographic research, the numerical model developed by Talukdar manages to meet these requirements (Talukdar, 2013). This supposes that such model achieves to predict with a significant precision the degradation process of the concrete under the consideration of the carbonation corrosion.
The potential of the model applied in this research to obtain the degradation curves is not only given by considering together the climatic and constructive parameters that govern the degradation, but
401 382.5 303.2 192.5 0 50 100 150 200 250 300 350 400 450 Europe
Rest of the World USA China
Cost of Corrosion (USD Billions)
Chapter 1: Introduction
10
also by considering the variability of these parameters (climatic) over time. Therefore, this feature fit perfectly in the context of climate change which is developed in this research.
These degradation curves are the basis for the formulation of optimal maintenance strategies for structural systems. These strategies include a decision-making model whose main objective is to prevent premature degradation and extend the life cycle of the structure. A decision support system is highly necessary to help engineers understand the benefits of real-time maintenance strategies. Proper repair and maintenance tasks to preserve the required performance or extend a specific service period for a building are essential for sustainable development in construction.
Figure 1.3 shows a diagram that represents the most critical aspects of the formulation of the maintenance strategy proposed by this research. The optimisation task seeks to cover a very sensitive and essential aspect within any constructive system, i.e., the economic perspective of a project. Many of the projects take into consideration the cost of the project about the design and construction of the construction system. Nevertheless, they leave aside the post process of every project: the monitoring and maintenance of the infrastructure.
Figure 1.3 Formulation scheme of the support decision model for the maintenance of structures.
This research seeks to make both users of the infrastructures and the professionals involved aware of the immeasurable value provided by the optimised planning of maintenance tasks. Therefore, the significance of the maintenance of the structures must be appreciated from the functional perspective, safety, and above all, from the economic point of view of the project.
1.3 Research Objectives
The main objective of this thesis is to develop an optimised methodology for the formulation of strategies corresponding to the maintenance of reinforced concrete structures subjected to the degradation induced by carbonation, considering the effects of climatic changes. The proposed research seeks to raise awareness about this problem through the formulation of the following specific objectives:
11 Analyse the numerical models developed in the literature for the study of
carbonation-induced degradation in concrete structures;
Identify the climatic parameters that affect the degradation mechanism studied and the respective projections estimated considering the effect of climate change;
Select a numerical model and perform the calibration of the parameters used to obtain the degradation curves adapted to the structures in Paraguay;
Develop a cost analysis of the inspection and maintenance of the structures;
Establish the inspection times for the structure throughout its service life through the optimisation of cost and quality of the inspections;
Establish the most appropriate inspection techniques to be applied to the structure throughout its service life, optimising the probability of damage detection of these;
Develop a dynamic model for making decisions associated with maintenance activities where structural reliability is guaranteed at the lowest possible cost;
Establish the appropriate repair methods in concrete structures considering the effectiveness of the repair in the improvement of the performance of the structure;
Determine optimal maintenance schedule for minimum costs associated with carbonation-induced corrosion damage in reinforced concrete structures.
This research seeks to create a support tool for engineers who work in the field of maintenance of existing buildings. Although the focus of the maintenance model is oriented to the climatic and constructive conditions of Paraguay, it is intended that the method can be emulated for other regions of the world. However, special care must be taken to the specific conditions that govern degradation for each case. Figure 1.4 shows a flowchart of the activities and objectives to be achieved with the elaboration of this research.
